1. Technical Field
The present disclosure is generally related to photoacoustic systems and methods and, more particularly, is related to aerosol photoacoustic systems and methods.
2. Description of the Related Art
Recent terrorist activities in the United States and elsewhere have demonstrated the susceptibility of both military and civilian personnel to chemical/biological aerosol attacks and the need to develop some type of early warning system. The majority of “point” sensors deployed in the field today fall into two basic categories: 1) chemical analysis on captured aerosol samples (very specific identification); and 2) an optical approach that relies on measuring the fluorescence phenomena inherent in most living things (very general discrimination). However, both approaches suffer from limitations. See J. Ho, “Real Time Detection of Biological Aerosols”, Defense Research Establishment Suffield, DRES R&D Bulletin 96001; P. Hairston, J. Ho, F. R. Quant, “Design of an Instrument for Real-Time Detection of Bioaerosols using Simultaneous Measurement of Particle Aerodynamic Size and Intrinsic Fluorescence”, J. Aerosol Sci. 28, no.3, 471, (1997).
Chemical methods that rely on various forms of antibody-tagging and/or chemical analysis often require periods on the order of several minutes in order for the chemical reaction to occur. In addition to the undesirable long periods of time needed to complete the analysis, these “wet chemistry” approaches generate quantities of toxic waste that require special handling and disposal.
Optical methods that are based on fluorescence operate in a more “real-time” manner; however, resultant measured fluorescent spectra is extremely sensitive to small quantities of contaminating media and/or particle morphology, resulting in false alarms. See G.W. Faris, R.A. Copeland, “Spectrally Resolved Absolute Fluorescence Cross Section of B. Globigii and B. Cereus”, Stanford Research Institute, Technical Report 2913, (1992); B. Bronk, L. Reinisch, “Variability of steady-state bacterial fluorescence with respect to growth conditions”, Appl. Spectroscopy, vol. 47, no. 4 pp. 436-440 (1993). When high-quality uncontaminated fluorescence spectra is recorded, it is often quite broad and featureless, making species discrimination and/or identification very difficult. See R. Pinnick, S. Hill, P. Nachman, “Aerosol fluorescence spectrum analyzer for rapid measurement of single micrometer-sized airborne biological particles”, Aerosol Science & Tech., vol. 28, no. 2, pp. 95-104, (1998).
A more desirable approach would be an optically based detection scheme that is sensitive enough to warn of the presence of a harmful airborne agent, but not so sensitive that it would result in many false positives, e.g., presence of residual growth media and changes in particle size or shape due to agglomeration.
Early development of aerosol photoacoustics was limited to single wavelength operation and, as a result, the method was thought to have little utility outside of the area of analytic spectroscopy. See C.W. Bruce, N.M. Richardson, “Propagation at 10 μm through smoke produced by atmospheric combustion of diesel fuel,” Appl. Opt., vol. 22, no. 7, pp. 1051-1056 (1983); C.W. Bruce, T.F. Stromberg, K.P. Gurton, “Trans-Spectral Absorption and Scattering of Electromagnetic Radiation by Diesel Soot,” Appl. Opt., vol. 30, no. 12, pp. 1537-1546 (1991). A brief description of the photoacoustic method is as follows.
Consider a sample volume containing a collection of optically absorbing particles, e.g., an aerosol. When a laser is passed through this volume a portion of the light will be absorbed by the aerosol particles, resulting in particle heating. Heat energy from the absorbing particle will transfer to the surrounding media and result in a pressure rise within the sample volume. If the laser is modulated at some convenient acoustic frequency, e.g., 1 kHz, a resultant acoustic signal will result at that same 1 kHz modulation frequency. This acoustic signal is then detected and amplified using a variety of means. It is established that the power of the resultant acoustic signal is directly proportional to the optical absorption cross-section of the absorbing particles. See A. Rosencwag, Photoacoustics and Photoacoustic Spectroscopy, Krieger Pub., New York (1990).
In principle, aerosol photoacoustics depends only on the fraction of light absorbed by the media and thus requires no a priori knowledge of aerosol particle shape, size, or complex refractive index. By designing a system in which the probe laser(s) also serve as a coincident short-pass transmissometer, an independent, absolute measure of the total extinction is also obtained. As a result, accurate values of the single-scatter albedo are also determined by taking the difference between the total extinction and absorption. See C. Bohren, D. Huffman, Absorption and Scattering of Light by Small Particles, Wiley-Interscience Press, New York, NY (1983).
Prior attempts at applying conventional photoacoustic spectroscopy to an aerosol media used a single monochromatic source and a “closed” sample volume geometry. See D.M. Roessler, F.R. Faxvog, “Optoacoustic measurement of optical absorption in acetylene smoke,” J. Opt. Soc. Am., vol. 69, no. 12, pp. 1699-1704, (1979). As a result, these methods were subject to problems associated with non-uniform aerosol concentrations due to, e.g., particle settling. In addition, these closed systems relied on optical windows to transmit the optical energy into the photoacoustic cell. Often, for moderately absorbing aerosols, the residual absorption due to the window substrate is comparable or greater than the absorption of the aerosol in question. See C.W. Bruce, R.G. Pinnick, “In-situ measurements of aerosol absorption with a resonate cw laser spectrophone,” AppL Opt., vol. 16, no. 7, pp. 1762-1765 (1977).
U.S. Patents relating to particle testing systems and methods include U.S. Pat. No. 6,694,799 issued to Small; U.S. Pat. No. 6,662,627 to Arnott et al.; U.S. Pat. No. 6,408,681 to Gurton et al.; U.S. Pat. No. 6,396,058 Gurton; U.S. Pat. No. 4,740,086 to Oehler et al.; U.S. Pat. No. 4,657,397 to Oehler et al.; U.S. Pat. No. 4,415,265 to Campillo et al.; and U.S. Pat. No. 4,187,026 to Schaffer et al., the teachings of each of which are fully incorporated herein by reference.